1. Field of Invention
This invention relates generally to mass flowmeters, and more particularly to a Coriolis-type meter in which the fluid to be metered is conducted through a straight flow tube which is excited to vibrate in a circular path.
2. Status of Prior Art
A mass flowmeter is an instrument for measuring the mass of a fluid flowing through a conduit per unit time. Most meters for this purpose measure a quantity from which the mass can be inferred, rather than measuring mass directly. Thus, one can measure the mass flow with a volumetric flowmeter by also taking into account pressure, temperature and other parameters to compute the mass.
A Coriolis-type mass flowmeter provides an output directly proportional to mass flow, thereby obviating the need to measure pressure, temperature, density and other parameters. In this type of meter, there are no obstacles in the path of the flowing fluid, and the accuracy of the instrument is unaffected by erosion, corrosion or scale build-up in the flow sensor.
In the Roth U.S. Pat. No. 3,132,512, a Coriolis-type mass flowmeter is disclosed in which a flow loop vibrating at its resonance frequency is caused to oscillate about a torque axis which varies with fluid flow in the loop. This torsional oscillation is sensed by moving coil transducers.
The Cox et al. U.S. Pat. No. 4,127,828 and U.S. Pat. No. 4,192,184 show a Coriolis-type meter having two U-shaped flow loops arranged to vibrate like the tines of a tuning fork, the torsional oscillation of these loops being sensed by light detectors to determine the mass flow. In the Smith U.S. Pat. No. 4,222,338, electromagnetic sensors provide a linear analog signal representing the oscillatory motion of a U-shaped pipe. Electromagnetic sensors are also used in the Smith et al. U.S. Pat. No. 4,492,025, in which the fluid whose mass is to be measured flows serially through two parallel U-shaped pipes.
The present invention provides a Coriolis-type mass flowmeter using a straight measuring tube. Of greatest prior art interest in this regard is the patent to Sipin, 3,329,019, which also discloses a straight measuring tube. In Sipin, this tube is electromagnetically excited to vibrate up and down, and because the tube is at the same time subjected to Coriolis forces, the vibrating tube oscillates torsionally. Strain gauge sensors are provided adjacent the inlet and outlet ends of the tube to sense the torsional oscillations.
The advantage of the straight tube arrangement of Sipin over measuring tubes disclosed in prior art Coriolis-type mass flowmeters is that the latter flowmeters employ U-shaped or otherwise curved flow tubes which somewhat impede flow, whereas a straight flow tube offers minimal resistance to flow and therefore exhibits the lowest possible pressure drop.
The drawback to the Sipin arrangement is that the measuring tube which is excited to vibrate up and down is sensitive to disturbances from external sources which act to impart vibratory motion to the flow tube, and these disturbances adversely affect the accuracy of the meter.